In OctoPi 0.15, you will also need to set the WiFi Country. The setting for this is also found in the /boot/octopi-wpa-supplicant.txt file. Scroll to near the bottom of the file and find the section that begins with "# Uncomment the country your Pi is in..." By default, the line for the United Kingdom is uncommented (the "#" is removed from the front of the line). If you are in some country other than the UK, comment out the United Kingdom line by putting a # in front of it, then remove the # from in front of the country you are in (do NOT remove the # from the middle of the line). If your country is not in the list, follow the link provided in the file to find the complete list, and create a new line for your country. For example, if you are in Switzerland, you would scroll down the page to find that the 2 letter code is "CH", and create a new line that looks like this: